1. Field of the Invention
The present invention relates to an imaging optical system, e.g., an imaging optical system that is installed in a mobile device (a smart phone, etc.) having a built-in camera.
2. Description of Related Art
Patent Literature Nos. 1 through 4 each disclose an imaging optical system installed in, e.g., a mobile device, having a built-in camera, which provides an f-number of approximately 2.0 through 2.4 and a half angle-of-view of 35 degrees.
The imaging optical system in each of Patent Literature Nos. 1 through 4 has a configuration of six lens elements, in which a positive lens element is provided closest to the object side, and a positive lens element or a negative lens element that has an aspherical surface having inflection points other than at the optical axis (at positions other than at an intersection point of the optical axis) is provided closest to the image side.
Patent Literature 1: Japanese Unexamined Patent Application No. 2014-44373
Patent Literature 2: Japanese Unexamined Patent Application No. 2014-52635
Patent Literature 3: Japanese Unexamined Patent Application No. 2013-242449
Patent Literature 4: Pamphlet of International Publication No. 2012/8357
However, since the imaging optical system in each of Patent Literature Nos. 1 through 4 has an inappropriate lens profile or refractive power in the positive lens element provided closest to the object side (and also an inappropriate refractive power balance between this positive lens element and the other lens elements), miniaturization (slimming down) of the imaging optical system is difficult, and there is a problem with the correction of abaxial aberration such as coma, etc., being insufficient. In particular, the slimming down of mobile devices having a built-in camera has gained considerable momentum, thereby demanding miniaturization (slimming down) of the imaging optical system to the utmost limit.